d and f Block Elements - Lanthanoids and Actinoids

The $f$-block elements are called inner transition elements because they involve the filling of the $(n-2)f$ subshell. They consist of two series: Lanthanoids ($4f$ series) and Actinoids ($5f$ series).

General electronic configuration of Lanthanoids is $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$ and for Actinoids it is $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$.

Lanthanoid Contraction: The steady decrease in the atomic and ionic radii (specifically $M^{3+}$ ions) with increase in atomic number. This is caused by the poor shielding effect of $4f$ electrons against the increasing nuclear charge.

Consequences of Lanthanoid Contraction: It leads to the similarity in the chemical properties of elements of the second ($4d$) and third ($5d$) transition series, such as $Zr$ and $Hf$ having nearly identical radii.

Oxidation States: The most common oxidation state for both series is $+3$. However, Lanthanoids exhibit $+2$ and $+4$ if they lead to stable $f^0, f^7$, or $f^{14}$ configurations (e.g., $Ce^{4+}$ is $f^0$ and $Eu^{2+}$ is $f^7$).

Actinoids show a wider range of oxidation states (up to $+7$) compared to Lanthanoids because the energy difference between $5f, 6d$, and $7s$ subshells is very small, allowing more electrons to participate in bond formation.

Actinoid Contraction: Similar to Lanthanoid contraction, but more pronounced due to even poorer shielding by $5f$ electrons compared to $4f$ electrons.

Chemical Reactivity: Lanthanoids are highly electropositive and resemble Calcium in reactivity. Actinoids are even more reactive, especially when finely divided, and many are radioactive (transuranic elements).